When a stationary induction electrical apparatus, such as a transformer and reactor, is electrically connected to an electric power system, a big transient magnetizing inrush current may flow. After a stationary induction electrical apparatus has been electrically connected to the system, alternating magnetic flux, of which initial value is a residual magnetic flux when the circuit breaker is opened, flows into the iron core, and the transient magnetizing inrush current flows when the magnetic flux exceeds the magnetic saturation point of the iron core, and as the residual magnetic flux becomes higher, the saturation degree of the iron core increases, and transient magnetizing inrush current increases.
In the case of a transformer, the transient magnetizing inrush current (multiplying factor with respect to the rated current of the first wave peak value) reaches several magnitudes, so it is necessary to take measures to prevent the malfunction of a ratio differential relay for transformer protection or an overcurrent relay for power receiving, which is installed to activate when a short circuit accident occurs at the secondary side of the transformer, or to take measures considering the design of the transfer so as to withstand the electromagnetic force generated by the transient magnetizing inrush current. The transient magnetizing inrush current has a waveform protruding in one direction (like a half-wave rectification), and therefore contains many higher harmonics, such as a second harmonic and fourth harmonic.
Hence [the transient magnetizing inrush current] temporarily becomes a higher harmonic current source, and it is necessary to take measures so that this [higher harmonic current source] does not affect a static capacitor and various AC filter equipment installed in the system. If the transient magnetizing inrush current is extremely high, the electric power system may be affected, such as an instantaneous drop in power system voltage.
In order to decrease the transient magnetizing inrush current, it is suggested to calculate the residual magnetic flux based on the voltage of each phase of the transformer, and consider the feed timing of the circuit breaker (phase) so as to cancel the residual magnetic flux of each phase, as shown in Patent Document 1 and Non-patent Document 1, for example.
An example of a conventional method for determining the residual magnetic flux of a stationary induction electrical apparatus is measuring the change of the induced voltage generated in the stationary induction electrical apparatus when power supply is interrupted, loading this voltage waveform into a computer, performing an integration operation to determine the magnetic flux waveform, and determining the residual magnetic flux from the difference between the final value of this magnetic flux waveform and the center value of the sinusoidal vibration of the magnetic flux waveform before opening (see Patent Document 2).
Another example of a method for determining the residual magnetic flux is a magnetic flux measuring method for a stationary induction electric equipment, comprising: opening detection means for detecting an opening time of a circuit breaker connected to a power supply circuit of a stationary induction electrical apparatus; voltage signal measurement means for storing voltage signals of the power supply circuit for a predetermined time, and measuring the voltage signals of the power supply circuit before and after opening of the circuit breaker; voltage signal offset removal means for calculating the offset of the voltage signal before opening of the circuit breaker and removing the offset from the voltage signal; voltage signal integration block detection means for deciding the end of integration based on the voltage signal after opening of the circuit breaker; voltage signal integration means for integrating voltage signals; magnetic flux signal calculation means for acquiring a magnetic flux signal by removing the offset of the voltage integration signal; and residual magnetic flux calculation means for calculating the residual magnetic flux value from the magnetic flux signal (see Patent Document 3).    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-204368    Patent Document 2: Japanese Patent Application Laid-Open No. 2000-275311    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-232840    Non-Patent Document 1: “Controlled Switching of Unloaded Power Transformers”, Electra No. 212, p. 38 (2004)
In the case of the method for measuring the magnetic flux proposed in Patent Document 2, the final value of the magnetic flux waveform acquired by integrating the voltage is not constant, but increases or decreases monotonously because of the influence of the DC offset component superimposed on the measurement system. In the case of the magnetic flux waveform shown in FIG. 2 of Patent Document 2, for example, the final value φ1 is increasing monotonously. If the DC offset component is high, the integration result may diverge in a short time. Hence in the case of the invention according to Patent Document 2, the magnetic flux is different depending on the timing of reading, and an accurate residual magnetic flux cannot be calculated.
It is the magnetic flux measuring method according to Patent Document 3 that proposes one solution to the problem of Patent Document 2. The magnetic flux measuring method according to Patent Document 3 proposes a method of eliminating the influence of the DC offset component superimposed on the measurement system by calculating the DC offset from the sinusoidal voltage waveform before the circuit breaker performs opening operation, and integrating the resulting waveform after subtracting this DC offset from the voltage waveform as shown in FIG. 2 in Patent Document 3. This offset of the magnetic flux waveform as a result of the integration is also removed in the same way to calculate the residual magnetic flux.
However a problem of the method of Patent Document 3 is that the period of integrating a voltage waveform, which is executed to calculate the magnetic flux waveform, is extremely limited. In other words, the integration operation is performed for a voltage waveform in 200 ms and 100 ms periods before and after opening of the circuit breaker respectively. The reason why the integration operation period is limited is because the voltage waveform to be the integration target for removing offset must be stored once in the storage media, such as a memory.
For example, a 9.6 kB/phase memory capacity is required to store digital data having a voltage waveform which was analog-digital converted at 16-bit 4800 Hz for one second, and a memory capacity several times this capacity is required to execute the residual magnetic flux calculation proposed by Patent Document 3. Considering the cost-to-performance of the device in terms of economy and practicality, the calculation period of the magnetic flux waveform, that can be calculated by the magnetic flux measuring method proposed by Patent Document 3, is at most several seconds.
If the magnetic flux waveform measurement period is limited like this, the magnetic flux measuring device may experience a practical problem. In the case of the synchronous switching controlgear for circuit breaker considering residual magnetic flux, as shown in Patent Document 1 and Non-patent Document 1, it is critical to accurately measure the residual magnetic flux at all times, and having a limitation in the magnetic flux waveform measurement period may cause a practical problem.
For example, when a circuit breaker having an inter-pole capacitor switches a stationary induction electrical apparatus without load, voltage appears in the stationary induction electrical apparatus terminal via the inter-pole capacitor even after the circuit breaker is opened. Therefore, as FIG. 11 of Non-patent Document 1 shows, a large voltage may be generated in the stationary induction electrical apparatus terminal by the transient voltage which is generated after opening when an accident occurs of an external circuit of the stationary induction electrical apparatus, such as an electric station bus, and the residual magnetic flux may change. Since it is impossible to predict when this change of residual magnetic flux will occur, the magnetic flux waveform must be measured at all times.
Even in the normal power supply interruption operation of the stationary induction electrical apparatus, including opening when an accident occurs, the transient phenomena of the voltage waveform after power supply is interrupted may continue for several tens of seconds, depending on the impedance of the stationary induction electrical apparatus and the impedance conditions of the peripheral circuits thereof, and in order to accurately measure the residual magnetic flux in such a case, it is necessary to measure the magnetic flux waveform for several tens of seconds.
In this way, limiting the magnetic flux waveform measurement period is a problem that must be solved when the magnetic flux measuring method for the stationary induction electrical apparatus is applied to the synchronous switching controlgear of a circuit breaker, considering the residual magnetic flux.